Is there a Rust equivalent of what bigInt is to python? [closed]

Yes there are. Search for "bigint" on crates.io and there are plenty of results. They are not in the standard library but that's nothing unusual for Rust. Rust's standard library is kept small intentionally to minimise binary size ease maintenance (among other reasons).

Because Rust is not Python, and there's no reason for it to "Pythonise". Rust is a systems programming language without GC and with a focus on performance. There are many ways you can go about implementing a BigInt type, and there is no universal "best" way to do it. The most performant implementation heavily depends on your particular use case, which again is why there are many crates you can choose from, depending on what you need exactly.